1. Technical Field of the Invention
The present invention relates to an image-pick up lens, which forms an image of an object onto an image-receiving surface.
2. Related Art
In recent years, camera modules for taking photos have begun to be incorporated in mobile terminals including mobile phones. Downsizing the camera modules is a prerequisite for these apparatuses in order to enhance their portability. In the meantime, regarding an image pickup device such as CCD and CMOS, a pixel having the size of approximately a few μm has become feasible, and an image pickup device with high-resolution and compact size can be realized. Thus, there is high demand for cost reduction as well as downsizing of an image-pick up optical system to be incorporated to such image pickup device. An optical system is expected to satisfy all requirements of compactness, low cost, high resolution and excellent optical performance, which could conflict with each other.
More specific requirements expected of the optical system may roughly be classified as follows:
Low cost (the system including as few lenses as possible; enabled to be formed of resin; and easily assembled)
Brightness (small Fno)
Compactness (particularly, the length from the lens edge to the image pickup device being short)
Wide angle of field (desirably, 30° or more)
Uniform illumination on an image surface (few eclipses/narrowing down the angle of incidence onto an image pickup device)
High resolution (appropriately corrected fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration)
If an optical system satisfying all the above requirements can be formed with few lenses, the range of applying the system will be broadened. Many kinds of lenses have been proposed so far; however, it has been very difficult to satisfy all the requirements by using only two lenses.
As for the two-lens structure, arranging “a negative lens and a positive lens” from the side of an object is advantageous in correcting aberrations, but the arrangement limits the size reduction. In order to shorten the overall length of the two-lens structure, it is desirable to arrange a pair of “a positive lens and a positive lens” or a pair of “a positive lens and a negative lens”. Moreover, in order to narrow down the angle of incidence onto the image pickup device, it is desirable to employ a ‘front-set stop’ type, which has an aperture stop at the side closest to the object. Among structures having been proposed so far, Japanese patent unexamined application laid open No. 01-245211 and No. 04-211214 disclose structures satisfying the above requirements.
FIG. 22 shows a sectional view of the disclosure in the unexamined patent application laid open No. 01-245211. The structure is composed of a first biconvex positive lens and a second negative meniscus lens having a concave surface on the image side. In this example, the second lens exhibits relatively stronger power and has a concave surface on the side of an object, so that the angle of incidence onto the image pickup device is likely to be wide. Consequently, it is difficult to enlarge the angle of field, and the angle of field of the example is approximately 20°, which is a relatively small value.
FIG. 23 shows a sectional view of the disclosure in the unexamined patent application laid open No. 04-211214. The structure is composed of an image-pick up lens 10 on the side of an object and a correction lens 20 on the side of an image. In this example, the image-pick up lens 10 on the side of the object takes charge of almost all systems power, and the both surfaces of the correction lens 20 on the side of the image are aspheric. Consequently, field aberration is corrected so as to maintain balance among aberrations that occur when enlarging the angle of field. In this configuration, the lens 10 on the object side is, in charge of the fundamental imaging function. Therefore, when the lens on the object side is configured with a single lens, there is a limit in taking balance of field aberrations only by using the correction lens 20 while narrowing down the angle of incidence onto an image pickup device. Moreover, it is also difficult to correct chromatic aberration effectively, and thus widening the angle is limited.
The present invention provides a low-cost bright image-pick up lens system of a small size which is short in the total length, wherein angle of field is beyond 30°; the angle of incidence onto an image pickup device can be narrowed down; and various aberrations are appropriately corrected.